Method and apparatus for synchronizing a data communication system to a periodic digital impairment

ABSTRACT

A pulse code modulation modem system is configured to transmit data from a first modem to a second modem over the public switched telephone network (PSTN). The PSTN employs robbed bit signaling (RBS) such that symbols affected by RBS arrive at the second modem in a periodic manner based on a period of six symbols. The modem system is configured such that signal segments are formatted and transmitted with six symbols per segment. After obtaining symbol synchronization, the second modem initializes a modulo-6 symbol counter such that the zero count corresponds to the first symbol of each received signal segment. Those symbols affected by RBS are identified and analyzed to determine optimized signal point constellations that may be used to compensate for the RBS on a symbol-by-symbol basis during subsequent encoding and decoding. Upon a loss of synchronization, the second modem resets its modulo-6 counter in response to the detection of the first symbol in a subsequent signal segment. In this manner, the modem system can again use the optimized signal point constellations determined previously.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.09/009,228, filed Jan. 20, 1998 now U.S. Pat. No. 6,023,493.

FIELD OF THE INVENTION

The present invention relates generally to synchronization techniquesused in data communication systems, e.g., modem systems, that transmitdata between remote locations. More specifically, the present inventionrelates to a synchronization technique for use with a data communicationsystem that introduces digital impairments, such as robbed bitsignaling, to a transmit signal.

BACKGROUND OF THE INVENTION

Digital communication systems, such as modem systems, are well known inthe prior art. Such systems typically employ timing recovery techniquesthat are utilized to recover the symbol rate at which the data istransmitted. Such systems also use synchronization techniques that areutilized to align the receiver clock with the transmitter clock.Synchronization signals are often transmitted near the beginning of a"handshaking" procedure, during a receiver training procedure, orperiodically during data transmission (to resynchronize the receivemodem with the transmit modem). The prior art is replete with varioustiming recovery and synchronization techniques; several timing recoveryschemes are discussed in Lee & Messerschmitt, DIGITAL, COMMUNICATION,pp. 737-764 (2d ed. 1996), the contents of which are incorporated hereinby reference.

The current 56 kbps modem systems take advantage of the digital natureof the public switched telephone network (PSTN). The theoretical maximumdata rate of 64 kbps per channel may not be realized in conventional 56kbps modem systems that employ robbed bit signaling (RBS) to facilitatecontrol signaling between network nodes. A particular data transmissionmay pass through a number of network nodes; each link may introduce RBSsuch that several bits are eventually robbed from the originallytransmitted data codewords. The modem system can partially compensatefor this type of digital impairment if it can identify the affected bitsor the affected codewords.

For a given link in a 56 kbps modem transmission, RBS typically removesthe least significant bit from the transmitted PCM codeword and replacesthe bit with signaling information. The robbed bits are commonly forcedto "ones" by the central offices, which effectively reduces the numberof signal points in the signal point constellations utilized to decodethe codewords affected by RBS. If the receiving modem can identify thosesymbols affected by the RBS, then different signal point constellationsmay be employed in a symbol-by-symbol basis to optimize performance andcompensate for the RBS. Accordingly, a technique for synchronizing thereceive modem to the RBS is needed to facilitate the application ofsignal point constellations to RBS-tainted symbols.

In current 56 kbps modem systems, RBS may occur in any given networklink. In addition, symbols may be transmitted as a "continuous" streamor arranged in a number of signal segments or data frames having aparticular number of symbols per segment or frame. The receive modem isconfigured to obtain and maintain synchronization with the transmitmodem or network clock for purposes of timing and proper decoding.Present systems may utilize a special synchronization signal format or asymbol counting technique to monitor occurrences of RBS on asymbol-by-symbol basis. However, such techniques may break down if thereceiver modem loses synchronization with the transmit modem. When sucha receiver modem regains proper synchronization, it may not be possibleto readily identify those symbols affected by RBS; the receiver modemmay apply RBS-correcting signal point constellations where suchcorrection is unnecessary. Untimely use of such corrective measures mayintroduce an undesirable number of decoding errors.

Therefore, a technique is needed to address the above disadvantages andshortcomings of prior art 56 kbps modem systems.

SUMMARY OF THE INVENTION

Accordingly, it is an advantage of the present invention that animproved synchronization procedure for use in a data communicationsystem is provided.

Another advantage of the present invention is that it provides amechanism for synchronizing a receive modem to a digital impairment,such as robbed bit signaling (RBS), that is introduced to the transmitsignal.

Another advantage is that the present invention provides for a modemsystem that obtains RBS synchronization upon receiver/transmittersynchronization.

A further advantage is that RBS synchronization may be easilyreestablished subsequent to a loss of receiver/transmittersynchronization.

Another advantage of the present invention is that the modem system canemploy corrective signal point constellations to encode and decodeRBS-tainted symbols in a reliable manner.

The above and other advantages of the present invention are carried outin one form by a synchronization method for use in a data communicationsystem having a first device configured to communicate with a seconddevice, where the communication network between the two devicesintroduces digital impairments to a transmit signal such that thedigital impairments arrive at the second device in a periodic mannerbased on a period of N symbols. The method may begin by transmitting aplurality of signal segments from the first device to the second device,where each of the signal segments is represented by an integer multipleof N symbols. The second device then obtains synchronization associatedwith symbols transmitted by the first device.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconnection with the Figures, where like reference numbers refer tosimilar elements throughout the Figures, and:

FIG.1 is a block diagram of an exemplary 56 kbps pulse code modulation(PCM) modern environment;

FIGS. 2-4 depict exemplary transmit signals with symbols affected byRBS;

FIG. 5 is a block diagram of an exemplary modem system in which thesynchronization techniques of the present invention may be incorporated;

FIG. 6 is a flow diagram of an RBS synchronization process that may becarried out by the modem system shown in FIG. 5; and

FIG. 7 is a flow diagram of a resynchronization process that may beperformed by the modem system shown in FIG. 5.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

FIG. 1 shows an exemplary 56 kbps pulse code modulation (PCM) basedmodem environment in which the present invention may operate. Aninternet service provider (ISP) or central site 100 is digitallyconnected to a telephone network 130 through a transmitter 110 and areceiver 120 resident at an ISP modem 124. Telephone network 130 isconnected to a local loop 150 through a central office line card 140.Line card 140 typically has a PCM codec (not shown) implemented therein.Local loop 150 is connected to a user's personal computer (PC) 170 atthe user's site through the user's modem 160. As can be appreciated bythose skilled in the art, the connection between transmitter 110 totelephone network 130 is a digital connection with a typical data rateof about 64 Kbps. Since the parameters of telephone network 130 and linecard 140 are dictated and set by the telephone company's specificationsand operation (and particularly their use of the μ-law signal pointconstellation), transmitter 110 needs to transmit the digital data in aparticular format to fully exploit its digital connection to telephonenetwork 130. Those skilled in the art will appreciate that the systemdepicted in FIG. 1 may employ any number of known signal processing,coding, and decoding techniques related to, e.g., μ-law signal pointconstellations, shell mapping, spectral control, equalizer training, andthe like. For the sake of brevity, such known techniques and systems arenot described in detail herein. It should also be noted that theprinciples of the present invention are not limited to modemapplications and that the present invention may be suitably modified orconfigured for deployment in any number of data communication systems.

Generally, the typical PCM modem system formats digital data fortransmission from ISP modem 124 to user modem 160, where the digitaldata is retrieved for use by PC 170. The data may be arranged into datasymbols and encoded via any number of techniques such as μ-law mapping.The data symbols may then be further processed with spectral control orother signal conditioning schemes prior to being transmitted as one ormore signal segments at a particular symbol rate to telephone network130. User modem 160 eventually receives the data symbols and thereafterdecodes the data to obtain the original digital data. To functionefficiently, user modem 160 should be synchronized with ISP modem 124.Consequently, user modem 160 may include a timing recovery scheme thatrecovers the transmitted symbol rate and synchronizes the receiver atuser modem 160 with transmitter 110.

Due to the digital nature of telephone network 130 and the design ofcurrent 56 kbps modem systems, digital impairments may be present withina data communication channel. For example, present 56 kbps modem systemsmay employ techniques to compensate for robbed bit signaling (RBS) whichmay occur in the digital links within telephone network 130. Although intheory 64 kbps may be allocated for any given channel, some of the bitsmay be "robbed" and replaced with data for use with control signaling ona link between network nodes. Each data symbol typically includes a7-bit PCM codeword and a sign bit; the robbed bits are periodicallytaken from the least significant bit positions associated with thetransmitted PCM codewords. The central office codec may assign "ones" tothe robbed bits for purposes of decoding. Thus, unless otherwisecompensated for, RBS can introduce decoding errors to the transmitteddata.

Any given communication channel may be susceptible to any number ofrobbed bits associated with multiple network links within telephonenetwork 130. For example, for a given network link, RBS may occur onceevery sixth PCM codeword, i.e., once every sixth symbol. On a subsequentlink, another symbol position may be affected by RBS. FIG. 1 depicts anexemplary signal segment 174 transmitted by ISP modem 124 (for purposesof this example, signal segment 174 is six symbols long). For theparticular end-to-end channel between ISP modem 124 and user modem 160,telephone network 130 may introduce RBS with respect to any number ofsymbols associated with signal segment 174. In this example, RBS hasoccurred at symbol positions 1 and 3, as indicated in signal segment176. It should be appreciated that the present invention is not limitedto any specific signal segment length and that the specific symbollocations associated with RBS may differ than that shown and describedherein.

In accordance with the present invention, telephone network 130 operatessuch that symbols affected by RBS arrive at user modem 160 in a periodicmanner based on a period of N symbols. Thus, for the example shown inFIG. 1, RBS would occur every six symbols for symbol positions 1 and 3.FIG. 2 depicts an exemplary transmit signal formatted as a continuousstream of symbols. As shown, the RBS occurring at the first symbolperiodically occurs every sixth symbol, i.e., at the seventh symbol, thethirteenth symbol, and so on. Likewise, the RBS occurring at the thirdsymbol periodically occurs every sixth symbol, i.e., at the ninthsymbol, the fifteenth symbol, and so on. The present invention takesadvantage of this periodic nature of the RBS by defining and formattingall transmitted signal segments such that they are represented by aninteger multiple of N symbols, where N is the period of RBS occurring atuser modem 160 (although not a requirement of the present invention, thepreferred exemplary embodiment described herein operates with N=6 forcompatibility with the RBS period found in current 56 kbps modemsystems).

FIG. 3 depicts an exemplary transmit signal defined by a plurality ofsignal segments, each having a length of six symbols. As shown, theperiodic nature of the RBS, in conjunction with the arrangement of thesignal segments, ensures that the symbol positions affected by the RBSare repeated for each six-symbol signal segment. In other words, theRBS-tainted symbols are the first and third symbols of each signalsegment. It should be noted that the principles of the present inventionare applicable to transmit signals that are defined by any integermultiple of N symbols, e.g., an integer multiple of six for the aboveexamples. FIG. 4 depicts another exemplary transmit signal defined by aplurality of twelve-symbol signal segments. In this example, theRBS-tainted symbols occur at the first, third, seventh, and ninthsymbols of each signal segment. In general, the present invention takesadvantage of this periodic characteristic to simplify thesynchronization process with respect to RBS and to effectively identifyand compensate for those symbols affected by RBS.

FIG. 5 is a block diagram of a preferred exemplary modem system 500 thatincorporates the synchronization features of the present invention, itwill be appreciated, however, that the present invention can beimplemented in combination with any number of different synchronization,timing recovery, and other signal processing techniques known in theart. For example, many suitable techniques are described in Lee &Messerschmitt, DIGITAL COMMUNICATION (2d ed. 1996), which isincorporated herein by reference. Accordingly, the particularimplementations shown and described herein are merely exemplary and arenot intended to limit the scope of the present invention in any way.Indeed, for the sake of brevity, various timing recovery, automatic gaincontrol (AGC). synchronization, training, decoding, and other techniquesemployed by modem system 500 may not be described in detail herein.

It should be appreciated that the functional blocks depicted in FIG. 5are merely exemplary and are not intended to represent physicallimitations of modem system 500. These, and other functional elements ofmodem system 500, may be implemented with any number of semiconductordevices, including memory elements configured to store data, functionalparameters, software instructions, and other information, andmicroprocessors configured to carry out the various processes describedherein. Furthermore, modem system 500 may include conventional hardwareor software elements configured to perform well known operations relatedto digital signal processing and/or data transmission; such elements arenot described in detail herein.

Generally, modem system 500 includes a first modem, e.g., modem 502, anda second modem, e.g., modem 504. Modems 502, 504 are generallyconfigured in accordance with known principles to communicate via thepublic switched telephone network (PSTN) 506. In current 56 kbps modemsystems, PSTN 506 typically introduces RBS to a number of transmittedsymbols, as described above.

First modem 502 may include a processor/controller 514 configured tocarry out various tasks associated with the operation of modem 502.Although not shown, modem 502 may incorporate additional processors orcontrol elements as necessary to suitably support its operation.Processor/controller 502 may suitably interact with other functionalcomponents of modem 502 to thereby access and manipulate data or monitorand regulate the operation of modem 502.

First modem 502 preferably includes an encoder 508 configured to encodedigital data in accordance with the particular encoding protocolemployed by modem system 500. For example, μ-law or A-law encodingtechniques are used in conventional modem systems in accordance withwell-established standards. The output signal generated by encoder 508may include information for transmission during a data mode,synchronization or training signals for transmission during aninitialization mode, or control or other signaling data employed bymodem system 500. A signal point constellation database 510 may beassociated with encoder 508, as depicted in FIG. 5. It should beappreciated that database 510 need not be an integral part of encoder508 and that modem 502 may implement database 510 in a different mannerthan that shown. As described in more detail below, database 510 may beemployed by modem 502 to store a plurality of signal pointconstellations associated with specific symbol positions within asegment. Such constellations are suitably used to facilitate thetransmission of signal segments from modem 502 to modem 504. Forexample, compensating signal point constellations may be derived andstored for use during encoding and decoding of signal points associatedwith RBS to optimize the performance of modem system 500 in the presenceof RBS.

Modem 502 includes a transmitter 512, which is configured to transmitencoded data symbols in accordance with general PCM techniques. Inaccordance with one aspect of the present invention, modem 502 isconfigured to transmit signal segments that have been formatted suchthat each signal segment is represented by an integer multiple of Nsymbols, where N is the period with which RBS-tainted symbols repeat atmodem 504 (as described above). Processor/controller 514 may beconfigured to perform such formatting to ensure that each signal segmentis a multiple of N symbols long. In addition, modem system 500 may begoverned by applicable standards such that certain signals, e.g.,predetermined training signals, control signals, and the like, aredefined to be a multiple of N symbols long. In accordance with oneaspect of the present invention, a period of silence may be transmittedby modem 502 in the form of one or more signal segments having amultiple of N "zero" symbols.

Modem 502 may employ a symbol or segment counter 516 to monitor therelative symbol positions associated with signal segments transmitted bymodem 502. Counter 516 may be employed to enable encoder 508 toassociate particular signal point constellations with different symbolcounts within a segment. In one embodiment where signal segments arerepresented by six symbols, counter 516 may be configured as a modulo-6counter. Counter 516 may be configured to function in accordance withany number of known digital signal processing techniques.

Modem 502 includes a receiver 518, which may be configured in accordancewith conventional modem technology. Receiver 518 is configured toreceive data from modem 504; such data may include encoded informationbits, control signals, functional parameters or identifiers, and anyother data employed by conventional modem systems. For example, and asdescribed in more detail below, modem 504 may be configured to sendinformation indicative of optimized signal point constellations to modem502 for use during transmission of subsequent signal segments. Ofcourse, modem 502 may employ any suitable alternative device ortechnique for receiving the optimized signal point constellations frommodern 504. A decoder 520 may be used to decode any signals transmittedby modem 504 to modem 502, including the signal that conveys theoptimized constellations.

Signal segments are suitably transmitted over a forward channel to modem504 in accordance with conventional transmission techniques. Asdescribed above, signal segments transmitted through PSTN 506 may besubject to RBS in any number of network links. Eventually, the signalsegments are received at a receiver 530 located at modem 504; modem 504processes the received signals to obtain the original digital dataencoded by modem 502. It should be noted that receiver 530 may includeany number of additional components (that may be known in the art) fordecoding, gain control, timing- recovery, equalization, conditioning, orother processing of the received signal.

Like modern 502, modem 504 may include a processor/controller 536configured to carry out various tasks associated with the operation ofmodem 504. Processor/controller 536 may suitably interact with otherfunctional components of modem 504 to access and manipulate data ormonitor and regulate the operation of modem 504. For example,processor/controller 536 may be configured to operate in conjunctionwith a decoder 532 to suitably decode the received symbols in accordancewith the same encoding scheme employed by encoder 508. Decoder 532 maybe configured in accordance with known signal processing techniques. Aswith encoder 508, decoder 532 may have a signal point constellationdatabase 534 associated therewith. Database 534 is preferably utilizedto store different signal point constellations that may be optimized foruse with particular symbol counts within a given signal segment. Inaccordance with the present invention, the same optimized signal pointconstellations may be used for encoding and decoding of individualsymbols when modem 504 is synchronized with modem 502 and when counters516 and 538 are aligned. This enables modem system 500 to effectivelycompensate for RBS or channel characteristics on a symbol-by-symbolbasis.

A symbol counter, such as a modulo-6 symbol counter 538, is preferablyutilized to enable modem 504 to correctly apply the appropriate signalpoint constellation to the received symbols. To enable synchronizedoperation of encoder 508 and decoder 532, processor/controller 536 mayinitialize or reset counter 538 in accordance with a synchronizationscheme 540 performed by modem 504. It should be noted thatsynchronization scheme 540 may perform any number of conventionalsynchronization techniques known to those skilled in the art and thatthe present invention is not limited to any particular synchronizationmethodology. In the preferred embodiment, synchronization scheme 540 maysuitably detect one or more synchronization signal segments transmittedby modem 502 during a start-up period or during a re-synchronizationprocedure.

After detection of such a synchronization signal, modem 504 may resetcounter 538 in response to receipt of an initial signal segmenttransmitted after the synchronization signal. For example, modem 504 mayreset counter 538 upon or after receipt of a first symbol of asubsequent signal segment that follows the initial segment.Alternatively, the resetting of counter 538 may be prompted by thedetection or receipt of any suitable reference-position symbol (i.e.,other than the first symbol) associated with the initial signal segmentor associated with a subsequent signal segment. Of course, use of adifferent reference symbol position may require additional processing tocompensate for any applicable offset between counters 538 and 516.Resetting counter 538 based upon a subsequent signal segment may bedesirable in applications where it is difficult to accurately orefficiently detect the first symbol of the initial signal segmentimmediately received after the synchronization signal. Eventually, whenmodem 504 is synchronized with modem 502, counter 538 will maintain asymbol count consistent with modem 502.

Once modem 504 is synchronized with modem 502, processor/controller 536may also serve to designate which signal point constellation should beused by decoder 532 to decode the current symbol. As described above,modem 502 preferably encodes data using signal point constellations(that may differ from one another) that have been optimized tocompensate for PBS. Consequently, to avoid introducing decoding errors,modem 502 and modem 504 should use the same constellations on asymbol-by-symbol basis. It should be noted that the constellations mayalso be optimized according to any number of channel characteristics,the particular configuration of receiver 530, or the use of digitalpads. In the context of the present invention, a "digital pad" may be acircuit configuration or a digitally implemented process applied to thetransmitted codewords that emulates the effect of an analog attenuator.For example, a digital pad may employ digital techniques such asconversion tables to transform a codeword representing a given signalpoint magnitude into a different codeword representing a reduced signalpoint magnitude.

As described above, modem system 500 preferably uses compensatingconstellations that are. optimized in response to the presence andeffects of RBS. Accordingly, modem 504 may include an RBSdetector/analyzer 540, which may be configured in accordance withconventional methodologies. RBS detector/analyzer 540 preferably detectsthe presence of RBS in a received signal and determines the symbolpositions, relative to each of the received signal segments, where RBSwas introduced by PSTN 506. RBS detector/analyzer 540 (or any suitablefunctional component resident at modem 504) may be further configured toderive, for each symbol position that may be affected by RBS, thecompensating signal point constellation for use during subsequentencoding and decoding of symbols located at that position. Thedetermination of the optimized signal point constellations may beperformed during transmission of one or more known training ordiagnostic signal segments (after modem 504 has been synchronized withmodem 502).

Once modem 504 has determined the preferred signal point constellationsfor use during subsequent data transmission, a transmitter 542 ispreferably utilized to send information indicative of the optimizedsignal point constellations to modem 502. In the preferred embodiment,the information transmitted by transmitter 542 is encoded prior totransmission over PSTN 506. Upon receipt of this information, modem 502performs decoding and processing to obtain the optimized signal pointconstellations for subsequent use by encoder 508.

FIG. 6 is a flow diagram of an RBS synchronization process 600 that maybe performed by a modem system, such as modem system 500. Process 600may be performed in addition to, or as an integral part of, one or moreother processes related to the overall functionality of modem system500. Furthermore, process 600 merely illustrates how the presentinvention may be implemented in an exemplary modem system, the specificnumber and ordering of tasks may not necessarily be as shown anddescribed herein.

Process 600 may begin with a task 602, during which modem 504 issynchronized to modem 502. In the preferred embodiment, a predeterminedsynchronization signal is transmitted from modem 502 during a start-upperiod; modem 504 may also be trained during this period. Although not arequirement of the invention, the synchronization signal may beformatted into segments each having N symbols, where N is a multiple ofthe RBS period as described above.

Following task 602, a task 604 may be performed to cause modem 502 totransmit a known signal having one or more signal segments, each signalsegment being represented by an integer multiple of N symbols. Forpurposes of this exemplary embodiment, N equals the RBS period of six(common to current 56 kbps modem systems). In one preferred embodiment,this known signal is configured in a pseudo-random manner, which enablesmodem 504 to train the equalizers (not shown) in receiver 530 and totrain the echo cancelers (not shown) in transmitter 512. Receiver 530may also utilize this known signal to analyze the effects of RBS, asdescribed in more detail below in connection with a diagnostic signal.Upon transmission of the first symbol of the pseudo-random signal,counter 516 (see FIG. 5) is preferably initialized (task 606) to beginmonitoring the symbol positions associated with each subsequent signalsegment.

After (or in response to) receipt of an initial segment associated withthe pseudo-random signal, a task 608 is performed to initialize counter538 associated with modern 504. As described above, the initializationof counter 538 may be delayed until one or more subsequent signalsegments are received at modem 504 to ensure that counter 538 isaccurately set to a zero state to identify the first symbol in a signalsegment. Consequently, this first symbol is associated with the samezero count maintained by counters 516 and 538 located at moderns 502 and504, respectively. In this respect, counter 538 may be considered to besynchronized or aligned with counter 516. As described above, counter538 is preferably a modulo-6 counter in the exemplary embodiment;counter 538 automatically resets itself after counting six symbols.Thus, as long as modem 502 continues to transmit signal segments havingsix symbols, counter 538 will properly track counter 516.

The above counting scheme is preferably maintained regardless of theinformation conveyed by each symbol. For example, the present inventionis configured such that periods of silence (e.g., symbols with all bitsset to zero, symbols corresponding to signal points having the lowestmagnitude relative to other signal points used in the currentconstellation, or the like) may be transmitted as one or more signalsegments (the only requirement is that the signal segment contain theappropriate number of symbols). In contrast, prior art systems that relysolely upon clock estimation techniques may not function effectivelyduring periods of silence. Such prior art systems may not be able toaccurately maintain a symbol count during long or arbitrary periods ofsilence. For example, to resume transmission after an arbitrary periodof silence, prior art systems may perform a resynchronization process toalign the receiver with the transmitter and a subsequentresynchronization process to align the RBS-framing with that of thetransmitter. A prior art system that employs symbol counters mustre-align the transmitter and receiver counters by, e.g., transmitting areference sequence to reset the counters or performing a suitablereinitialization procedure.

In contrast to such prior art systems, if clock synchronization has beenmaintained by modem system 500 throughout a period of silence, then nofurther synchronization processes are required because counters 516 and538 will have preserved their alignment during the silence. If, however,clock synchronization is not maintained, then receiver 530 may firstresynchronize its clock, then align counter 538 to counter 516 bydetecting a transition from one segment to another using the techniquesdescribed below in connection with process 700.

After counter 538 has been initialized, a diagnostic signal containingat least one signal segment may be transmitted from modem 502 to modem504 (task 610). Each diagnostic signal segment is formatted inaccordance with the above principles; in this example, each diagnosticsignal segment is a multiple of six symbols long. It should beappreciated that any diagnostic signal having suitable characteristicsmay be transmitted during task 610. For example, the preferredembodiment employs a diagnostic signal that contains most of the μ-lawcodewords likely to be used for the particular transmission. Thus, anexemplary diagnostic signal may include more than 100 symbolsrepresenting various constellation signal points. It should be notedthat the particular format of the diagnostic signal may vary from systemto system. The diagnostic signal is received by modem 504 and analyzedduring a task 612.

During task 612, RBS detector/analyzer 540 (see FIG. 5) suitably detectsthe effect of RBS in the received signal segments and determines thesymbol positions (if any) where RBS was introduced to the signal. Due tothe periodic nature of the RBS, detector/analyzer 540 need onlydetermine the RBS-affected symbol positions in any one signal segment;all other signal segments should be similarly affected. Task 612 mayemploy any number of conventional detection schemes known to thoseskilled in the art. Counter 538 may be accessed or monitored during task612 to suitably identify the symbol count associated with RBS-taintedsymbols. A task 614 may also be performed to derive optimized signalpoint constellations for use during subsequent encoding and decoding ofsymbols processed by modem system 500. One form of optimizedconstellation is a compensating constellation for use during encodingand decoding of symbols affected by RBS. Other optimized constellationsmay be derived in response to conventional line probing techniques thatdetermine transmission characteristics of the current channel. Task 614is preferably performed to derive a constellation for each symbolposition associated with the formatted segment length, e.g., six for theexample described herein. The different signal point constellations forthe various symbol positions may be stored in database 534. It should benoted that modem system 500 need not employ a unique signal pointconstellation for each symbol position.

For purposes of the present invention, task 614 may "drive" theoptimized signal point constellations by analyzing the RBS informationobtained in connection with the diagnostic signal and selecting from agroup of predetermined constellations. In this manner, any number ofoptimized signal point constellations can be stored at modem 504; suchstored constellations may be optimized in accordance with any number offactors such as the transmit power level, the use of digital pads andthe amount of attenuation caused by the digital pads, the location ofdigital pads relative to where RBS is introduced, and other knownsources of digital impairments. Accordingly, modem system 500 need notindividually determine new signal point constellations for each transmitsession.

Following task 614, a task 616 may be performed to cause modem 504 tosend information indicative of the optimized signal pointconstellations, and their corresponding symbol positions, to modem 502.Processor/controller 536 may be configured to format the data into aform suitable for encoding and transmission by transmitter 542. Suchtechniques are generally known in the digital signal processing field.The information is eventually received by receiver 518 at modem 502,decoded by decoder 520, and appropriately processed. In the preferredembodiment, the particular signal point constellations, with theircorresponding symbol positions, are stored in database 510 forsubsequent use during encoding.

A task 618 may be subsequently performed to cause modem system 500 toemploy the compensating signal point constellations in the periodicmanner with which RBS is introduced into the transmit signal. In otherwords, if the current channel causes RBS to be present in the first andthird symbol positions when received at modem 504, then the appropriatesignal point constellations "assigned" to the first and third symbolpositions will be used whenever those particular symbols are encoded anddecoded. As described above, other optimized constellations may beemployed in this periodic manner, whether or not they compensate for thepresence of RBS.

The present invention takes advantage of the periodic nature of RBSwithin signal segments received at modem 504 to effectively compensatefor the RBS. If modem 504 remains synchronized with modem 502, then theoccurrence and affect of RBS within a signal segment can be predictableon a symbol-by-symbol basis (as long as the signal segments areconsistently arranged to have the specified number of symbols). However,in practical modem systems, synchronization may be lost from time totime. Prior art modem systems may require a reanalysis of the channelcharacteristics or the effects of RBS upon resynchronization toredetermine the individual signal point constellations. In contrast, inaccordance with one aspect of the present invention, if modem 504 losesand thereafter regains synchronization, the symbol locations affected byRBS remain unchanged and the same optimized signal point constellationsmay be utilized as though synchronization was never lost. This featureenables modem system 500 to regain an optimized transmission mode in areduced amount of time compared to prior art modem systems.

FIG. 7 is a flow diagram that depicts a resynchronization process 700that may be performed by modem system 500 when modem 504 losessynchronization with modem 502. The following description of process 700assumes that modem 502 maintains "synchronization" with its symbolcounter 516 and/or a network counter (not shown) at all times. Process700 may begin with a task 702, during which modem 504 regains symbolsynchronization with modem 502. Those skilled in the art will recognizethat task 702 may be initiated by modem 504 upon a determination thatthe initial synchronization has been lost. Various renegotiation,signaling, and conventional symbol resynchronization procedures may beperformed by modem system 500 to enable modem 504 to regain symbolsynchronization with modem 502.

Following task 702, the symbol timing of modem 504 is preferably alignedwith modem 502.

However, the respective symbol counters 538 and 516 may not be operatingin a synchronized manner. If symbol counters 538 and 516 are notaligned, then the various optimized signal point constellations will notbe applied in a consistent manner during encoding and decodingAccordingly, a task 704 is preferably performed to cause modem 502 totransmit one or more repetitions of a known signal segment (having sixsymbols in accordance with the above example). In an exemplaryembodiment, this known signal segment may be similar to a V.34Modulation Parameter (MP) sequence or any suitable segment recognizableby modem 504. Task 704 may be performed in response to the proceduredescribed above in connection with task 702.

Modem 504 eventually receives the known signal segment and determinesits significance as a reference sequence. Next, a task 706 is preferablyperformed, during which modem 504 suitably identifies areference-position symbol of the known signal segment (or areference-position symbol of a subsequent signal segment). In thepreferred embodiment, task 706 identifies the first symbol of the knownsignal segment. Modem 504 may employ conventional techniques to performtask 706. For example, processor/controller 536 (see FIG. 5) mayrecognize the known signal segment and, consequently, determine theappropriate symbol count corresponding to the symbols in the knownsignal segment.

Preferably, after modem 504 identifies the first symbol of the knownsignal segment, a task 708 is performed. Task 708 resets counter 538 inresponse to the identification of the first symbol of the known signalsegment. In other words, task 708 causes counter 538 to again associatethe first symbol of received signal segments with a zero count. Thus,following completion of task 708, counter 538 is consistent with counter516 and modem 504 is resynchronized with modem 502 with respect tosymbol timing and monitoring of symbol positioning.

Following task 708, process 700 may end and modem system 500 may proceedin accordance with the above principles. Notably, the same optimizedsignal point constellations obtained prior to the loss of initialsynchronization can be utilized during subsequent encoding and decoding.This enables modem system 500 to take advantage of the consistentcharacteristics of the communication channel and the effects of RBS,neither of which should substantially vary during a given communicationsession over the same end-to-end link. This aspect of the presentinvention allows modem system 500 to efficiently establish a normaloperating mode without having to perform a lengthy reanalysis of the RBSsymbol positions or a redetermination of the optimized signal pointconstellations. It should be appreciated that, during such renegotiationprocedures, modem system 500 may be configured to alter the data rate,which results in the use of signal point constellations having more orless signal points than before the renegotiation. In such a situation,modem system 500 may utilize the RBS information obtained during theprevious analysis and apply such information to derive or select theappropriate signal point constellations for use with subsequenttransmissions at the new data rate. Again, the leveraging of thepreviously obtained information enables modem system 500 to efficientlydetermine optimized signal point constellations without having to repeatthe entire RBS analysis.

In summary, the present invention provides an improved synchronizationprocedure for use in a modem system. The present invention provides amechanism for synchronizing a receive modem to a digital impairment,such as RBS, that is introduced to the transmit signal. In accordancewith one aspect of the invention, a modem system can readily obtain RBSsynchronization upon synchronization of the receive modem with thetransmit modem. In addition, RBS synchronization and a corresponding RBScompensation technique may be easily reestablished subsequent to a lossof synchronization between the receive modem and the transmit modem. Anexemplary modem system employs corrective signal point constellations toencode and decode RBS-tainted symbols in a reliable manner.

The present invention has been described above with reference to variousexemplary embodiments. However, those skilled in the art will recognizethat changes and modifications may be made to the preferred embodimentswithout departing from the scope of the present invention. For example,the particular signal segment format may differ than that describedherein. In addition, various signal segments have been referred to(e.g., synchronization signal, subsequent signal segment, initialsegment, MP sequence) for the sake of convenience and to better describethe invention. However, those skilled in the art will appreciate thatthese designations in no way limit the practical application of theinvention. These and other changes or modifications are intended to beincluded within the scope of the present invention, as expressed in thefollowing claims.

What is claimed is:
 1. A synchronization method for use in a datacommunication system having a first device configured to communicatewith a second device over a communication network, said communicationnetwork introducing a number of digital impairments to a transmit signalsuch that said digital impairments arrive at said second device in aperiodic manner based on a period of N symbols, said method comprisingthe steps of:transmitting a plurality of signal segments from said firstdevice to said second device, each of said signal segments beingrepresented by an integer multiple of N symbols; obtainingsynchronization with symbols transmitted by said first device, saidobtaining step being performed by said second device; determining asymbol position, relative to each of said signal segments where adigital impairment was introduced by said communication network, saiddetermining step being responsive to said obtaining step; and deriving acompensating signal point constellation for use during subsequentencoding and decoding of signal points associated with said symbolposition, said deriving step being performed by said second device.
 2. Amethod according to claim 1, wherein said digital impairment comprisesrobbed bit signaling (RBS).
 3. A method according to claim 1, wherein atleast one of said signal segments is a period of silence.
 4. A methodaccording to claim 1, wherein N=6.
 5. A method according to claim 1,wherein said transmitting step transmits signal segments having Nsymbols.
 6. A synchronization method for use in a data communicationsystem having a first device configured to communicate with a seconddevice over a communication network, said communication networkemploying robbed bit signaling (RBS) such that symbols affected by RBSarrive at said second device in a periodic manner based on a period of Nsymbols, said method comprising the steps of:transmitting a plurality ofsignal segments from said first device to said second device, each ofsaid signal segments being represented by an integer multiple of Nsymbols; receiving said signal segments at said second device;synchronizing said second device to symbols transmitted by said firstdevice; and initializing a modulo-N symbol counter at said second devicein response to receipt of an initial signal segment by said seconddevice, said modulo-N symbol counter being synchronous to a symbol countassociated with said signal segments.
 7. A method according to claim 6,further comprising the steps of:detecting the effect of RBS in at leastone of said received signal segments, said detecting step beingperformed by said second device; and determining at least one symbolposition, relative to said at least one of said received signalsegments, where RBS was employed by said communication network.
 8. Amethod according to claim 7, further comprising the step of deriving,for each of said at least one symbol position, a compensating signalpoint constellation for use during subsequent encoding and decoding ofsymbols affected by RBS.
 9. A method according to claim 6, wherein N=6.10. A method according to claim 6, wherein said transmitting steptransmits signal segments having N symbols.
 11. A method according toclaim 6, wherein said initializing step initializes said modulo-N symbolcounter in response to receipt of a first symbol of a subsequent signalsegment that follows said initial signal segment.
 12. A method accordingto claim 6, wherein said initializing step initializes said modulo-Nsymbol counter after receipt of a reference-position symbol of saidinitial signal segment.
 13. A data communication system having a firstdevice configured to transmit signal segments to a second device over acommunication network, said communication network introducing a numberof digital impairments to a transmit signal such that said digitalimpairments arrive at said second device in a periodic manner based on aperiod of N symbols, said data communication system comprising:means forformatting a plurality of signal segments such that each of said signalsegments is represented by an integer multiple of N symbols; means fortransmitting said plurality of signal segments from said first device tosaid second device; means for synchronizing said second device tosymbols transmitted by said first device; and means for determining atleast one symbol position, relative to each of said signal segments,where a digital impairment was introduced by said communication network;wherein:said second device is configured to derive at least onecompensating signal point constellation for use during subsequentencoding and decoding of signal points associated with said at least onesymbol position; and said first device is configured to employ said atleast one compensating signal point constellation in said periodicmanner with respect to said at least one symbol position.
 14. A datacommunication system according to claim 13, wherein said digitalimpairment comprises robbed bit signaling (RBS).
 15. A datacommunication system according to claim 13, wherein N=6.
 16. A datacommunication system according to claim 13, wherein said means fortransmitting is configured to transmit signal segments having N symbols.17. In a data communication system having a first device configured totransmit data to a second device, a method for compensating for digitalimpairments in a transmit signal, where said digital impairments arriveat said second device in a periodic manner based on a period of Nsymbols, said method comprising the steps of:formatting a plurality ofsignal segments such that each of said signal segments is represented byan integer multiple of N symbols; starting a first symbol counter atsaid first device upon transmission of a first symbol associated withone of said signal segments; transmitting said signal segments from saidfirst device to said second device; initializing a second symbol counterat said second device after receipt of said first symbol by said seconddevice; and utilizing compensating signal point constellations, inresponse to said second symbol counter, for symbols affected by digitalimpairments.
 18. A method according to claim 17, further comprising thestep of determining at least one symbol position, relative to each ofsaid signal segments, where a digital impairment was introduced, saiddetermining step being responsive to said initializing step.
 19. Amethod according to claim 17, wherein said digital impairment comprisesrobbed bit signaling, (RBS).
 20. A method according to claim 17, furthercomprising the step of selecting a plurality of optimized signal pointconstellations associated with different states of said second symbolcounter, said selecting step being performed by said second device. 21.A method according to claim 20, further comprising the step of sendinginformation indicative of said optimized signal point constellationsfrom said second device to said first device.
 22. A method according toclaim 17, wherein said second symbol counter comprises a modulo-N symbolcounter.
 23. A method according to claim 17, wherein:said first symbolis associated with an initial signal segment; and said initializing stepinitializes said second symbol counter in response to receipt of a firstsymbol of a subsequent signal segment that follows said initial signalsegment.